One purpose of this study is to quantify the effect of a 50% reduction of sympathetic activity produced by guanadrel and simulated microgravity taking advantage of state-of-the-science techniques including measurements of norepinephrine spillover and clearance and direct measurements of sympathetic nerve traffic with microneurography. Another purpose is to evaluate whether a different model of weightlessness using guanadrel mimics the biochemical and physiological alterations characteristic of microgravity environment. We also plan to assess the role of sympathetic tone in modulating potassium channel blocker-induced QT interval prolongation.